Various publications, including patents, published applications, technical articles and scholarly articles are cited throughout the specification. Each of these cited publications is incorporated by reference herein, in its entirety and for all purposes.
Drug absorption is the sum total of the effects of various mechanisms by which drugs pass from the point of entry into the bloodstream. The rate and efficiency of drug absorption affects the rate and extent to which a drug reaches its intended site of action. Gastrointestinal absorption of orally administered drugs is, in part, a function of the permeability of mucosa in the gastrointestinal tract, particularly in the intestines, and also, in part, a function of the transit rate through the various organs of the gastrointestinal tract as the transit rate establishes the length of time the drug is localized to an absorption site.
Intestinal absorption of drugs can occur via different routes. Many orally administered drugs are absorbed by passive transcellular diffusion through the cell membrane of enterocytes (Van Asperen J et al. (1998) Pharm. Res. 37:429-35) or by passive paracellular diffusion through the tight junctions in the intestinal epithelium (Watson C J et al. (2001) Am. J. Physiol. Cell Physiol. 281:C388-C397). Epithelial drug absorption is also mediated by membrane transport proteins (Lee V H (2001) J. Natl. Cancer Inst. Monogr. 29:41-4). Such transport proteins can also serve as an impediment to drug absorption.
Various efflux transporters have been described, including members of the multidrug resistance protein (MRP) family (Borst P et al. (2000) J. Natl. Cancer Inst. 92:1295-1302), P-glycoprotein (P-gp) (Germann U A (1996) Eur. J. Cancer 32A:927-44), and breast cancer resistance protein (BCRP) (Staud F et al. (2005) Int. J. Biochem. Cell Biol. 374:720-5; Doyle L A et al. (1998) Proc. Natl. Acad. Sci. USA. 9526:15665-70), among others. Such efflux transport proteins are believed to be primarily responsible for low or variable absorption of orally administered drugs (Stephens R H et al. (2001) J. Pharmacol. Exp. Ther. 296:584-91).
Drug absorption and the factors that facilitate or impede it are thus important considerations in drug design and the evaluation of lead compounds as potential therapeutic agents. Several models are available for assessing absorption in the intestine. These models include the parallel artificial membrane permeability assay (PAMPA), in situ intestinal recirculating and single-pass perfusion, Ussing chambers, and cell lines, including Madin-Darby canine kidney cells (MDCK), and Caco-2 cells (Balimane P V et al. (2006) AAPS J. 8:E1-13).
Caco-2 cells, which were derived from a human colon adenocarcinoma, are a widely used model for intestinal absorption studies. When grown and allowed to differentiate, Caco-2 cells are morphologically similar to enterocytes and express many of the enzymes present in the small intestinal brush border, and thus closely resemble the environment and functions of the small intestine. Caco-2 cells provide an additional advantage for intestinal absorption studies in that they express at least three drug efflux transporter proteins, including P-gp (Hunter J et al. (1993) J. Biol. Chem. 268:14991-7), MRP proteins (Hirohashi T et al. (2000) J. Pharmacol. Exp. Ther. 292:265-70; Gutmann H et al. (1999) Pharm. Res. (NY) 16:402-7), and BCRP (Xia C Q et al. (2005) Drug Metab. Dispos. 33:637-43).
For drug absorption studies, it is desirable to evaluate the contributions of drug efflux transport proteins to impaired absorption. This can be accomplished by inhibiting the expression or activity of the transporters, particularly P-gp. In general, chemicals such as cyclosporine A are used to block the activity of P-gp. Chemical inhibition of transporters presents a disadvantage insofar as such chemicals also inhibit other cellular proteins and functions, and thus can skew the results of absorption experiments. Recently, the expression of P-gp in Caco-2 cells was shown to be reduced using RNAi technology (Watanabe T et al. (2005) Pharm. Res. 22:1287-93), and the expression of multidrug resistant gene 1 (MDR1) in Caco-2 cells was shown to be reduced using RNAi technology (Celius T et al. (2004) Biochem. Biophys. Res. Comm. 324:365-71). Inhibition of P-gp expression by RNAi enhanced the intracellular accumulation of and restored the sensitivity to compounds transported by P-gp (Wu H et al. (2003) Cancer Res. 63:1515-19). While genetically down-regulating P-gp in Caco-2 cells represents an improvement over the use of chemical inhibitors, studies of drug absorption in this model are disadvantaged in that the knockdown of P-gp expression alone does not account for contributions of extant transporters such as MRP and BCRP to drug efflux and impaired absorption. In addition, shRNA synthesized in vitro and directly transfected into cells reduces gene expression only transiently, and expression is restored a few days after transfection. Moreover, in vitro-synthesized shRNA is also often limited to cells that are easily transfected, and very little is known about the stability of inhibition of gene expression after several cell passages.
To accurately evaluate and predict the intestinal absorption of lead compounds, it is desired that the relative contributions of any and all efflux transport proteins be accounted for and controlled. Similarly, it is desirable to produce and utilize stable cell lines to genetically control the expression and/or function of such transport proteins on a more permanent basis. The present invention addresses these long-felt needs.